Environmental Research 119 (2012) 53–63
Contents lists available at SciVerse ScienceDirect
Environmental Research
journal homepage: www.elsevier.com/locate/envres
A screening model analysis of mercury sources, fate and bioaccumulation
in the Gulf of Mexico$
Reed Harris a,n, Curtis Pollman b, David Hutchinson a, William Landing c, Donald Axelrad d,
Steven L. Morey e, Dmitry Dukhovskoy e, Krish Vijayaraghavan f
a
Reed Harris Environmental Ltd., 180 Forestwood Drive, Oakville, Ontario, Canada L6J4E6
Aqua Lux Lucis, Inc., 8411 NW 55th PL, Gainesville, FL 32653, USA
c
Florida State University, Department of Earth, Ocean, and Atmospheric Science, 117 N. Woodward Ave., Tallahassee, FL 32306-4320, USA
d
Florida Department of Environmental Protection, 2600 Blair Stone Road, MS-6511, Tallahassee, FL 32399-2400, USA
e
Florida State University, Center for Ocean-Atmospheric Prediction Studies, Tallahassee, FL 32306-2840, USA
f
ENVIRON International Corporation, 773 San Marin Drive, Suite 2115, Novato, CA 94998, USA
b
a r t i c l e i n f o
abstract
Available online 25 October 2012
A mass balance model of mercury (Hg) cycling and bioaccumulation was applied to the Gulf of Mexico
(Gulf), coupled with outputs from hydrodynamic and atmospheric Hg deposition models. The dominant
overall source of Hg to the Gulf is the Atlantic Ocean. Gulf waters do not mix fully however, resulting in
predicted spatial differences in the relative importance of external Hg sources to Hg levels in water,
sediments and biota. Direct atmospheric Hg deposition, riverine inputs, and Atlantic inputs were each
predicted to be the most important source of Hg to at least one of the modeled regions in the Gulf.
While incomplete, mixing of Gulf waters is predicted to be sufficient that fish Hg levels in any given
location are affected by Hg entering other regions of the Gulf. This suggests that a Gulf-wide approach is
warranted to reduce Hg loading and elevated Hg concentrations currently observed in some fish
species. Basic data to characterize Hg concentrations and cycling in the Gulf are lacking but needed to
adequately understand the relationship between Hg sources and fish Hg concentrations.
& 2012 Published by Elsevier Inc.
Keywords:
Mercury
Methylmercury
Gulf of Mexico
Modeling
Mass balance
1. Introduction
The primary source of methylmercury (MeHg) exposure for most
North Americans is consumption of marine and estuarine fish
(Sunderland, 2007). Gulf of Mexico (Gulf) fisheries account for 41% of
the U.S. marine recreational fish catch and 16% of the nation’s marine
commercial fish landings (NOAA, 2011). While fish consumption has
well established health benefits (Mahaffey et al., 2011), concerns exist
regarding elevated MeHg levels observed in some fish species. MeHg is a
toxic and bioaccumulative form of mercury (Hg) with neurotoxicological
and cardiovascular effects in humans if exposure is excessive (Mergler
et al., 2007). Lowery and Garrett (2005) reported Hg concentrations of
0.15 to 4.0 mg g 1 wet muscle in Gulf king mackerel (n¼59; length 64
to 129 cm) and a standardized concentration of 1.3 mg g 1 wet muscle
at a length of 100 cm. All five Gulf States (Texas, Louisiana, Mississippi,
Alabama and Florida) have ‘‘do not eat’’ advisories for this species for
$
This research has not involved human subjects or experimental animals.
Corresponding author.
E-mail addresses: [email protected] (R. Harris),
[email protected] (C. Pollman), [email protected] (W. Landing),
[email protected].fl.us (D. Axelrad), [email protected] (S.L. Morey),
[email protected] (K. Vijayaraghavan).
n
0013-9351/$ - see front matter & 2012 Published by Elsevier Inc.
http://dx.doi.org/10.1016/j.envres.2012.08.013
women of child-bearing age and children (US EPA, 2003). Florida lists
over 60 Gulf species in its fish consumption advisory regarding mercury.
While Gulf fisheries are important regionally and nationally,
our understanding of factors controlling MeHg levels in the Gulf
food web is lacking. This is due to a combination of knowledge
gaps regarding Hg cycling in the oceans generally (e.g. where does
methylation occur, why do some marine fish have elevated MeHg
levels despite very low MeHg concentrations in water) and a basic
lack of Hg and MeHg concentration data for water, sediments and
the lower food web in the Gulf.
Mercury loading rates also affect fish Hg concentrations
(Munthe et al., 2007; Harris et al., 2007), but have not been
quantified for the Gulf. Atmospheric, terrestrial and Atlantic
inputs supply Hg that is redistributed to Gulf regions via large
scale water circulation patterns, including the Loop Current that
enters through the Yucatan Channel and exits through the Straits
of Florida without fully mixing with Gulf waters. There is a need
to identify which Hg sources are most important to the Gulf as a
whole, how Hg is redistributed, and whether the relative importance of Hg sources varies among locations, in terms of contributions to fish Hg levels. Without this information decision makers
will lack a sound foundation to assess the potential benefits of Hg
control strategies.
54
R. Harris et al. / Environmental Research 119 (2012) 53–63
Here we present the results of a screening level model analysis
to examine the following questions regarding Hg cycling and
bioaccumulation in the Gulf: (1) What are the key sources of
inorganic Hg and MeHg, (2) how does the importance of Hg
sources vary spatially, and (3) what spatial scale needs to be
considered when planning actions to reduce fish Hg levels in
different Gulf regions? This paper is a companion document to
Harris et al. (this issue), which more broadly reviews Hg sources,
transport, fate, bioaccumulation and human exposure in the Gulf.
2. Methods
The Gulf has an area of approximately 1.6 million km2 and a maximum depth
of 4 km in the Sigsbee Deep (UNEP, 2009; US EPA, 2011). While the five Gulf
States each have jurisdiction over a portion of Gulf waters, 94% of the area of U.S.
Gulf waters is in federal jurisdiction. A large continental shelf represents about
30% of the total area. There are 47 major estuaries (UNEP, 2009) contributing to
salinity gradients, particularly in the northern coastal areas, with large annual
cycles in the coastal freshwater budget associated with discharges from the
Mississippi and Atchafalaya Rivers (Morey et al., 2003). The Mississippi drainage
basin has an area of 3.2 million km2, nearly two-thirds of the total drainage basin
for the Gulf of Mexico, and 41% of the contiguous continental United States area
(US EPA, 2010). The largest source of water to the Gulf of Mexico, however, is the
inflow through the Yucatan Channel (8.6 1014 m3 yr 1), which is approximately
three orders of magnitude greater than the water load from the Mississippi
drainage basin. Much of this Atlantic inflow remains confined to the Loop Current
which exits the Gulf through the Straits of Florida. Dissolved organic carbon is on
the order of 1 mg L 1 in open waters in the Gulf (Baskaran et al., 1996; Guo et al.,
1995; Del Castillo et al., 2000) and pH is in the alkaline range ( 8, Solomon et al.,
2007). The Gulf is a moderately high productivity system (150–300 g C m 2 yr 1)
although conditions range from eutrophic in some coastal waters to oligotrophic
in deep water areas (UNEP, 2009). A large hypoxic area ( 15,000–20,000 km2)
forms in summer in bottom waters over a portion of the northern shelf.
A combination of field data and modeling was used to estimate Hg loading,
transport, transformation, fate and bioaccumulation to the Gulf as a whole and
within Gulf regions. Water circulation and atmospheric Hg deposition were
simulated with process-based mass balance models. A conceptually similar
approach involving coupled 3D hydrodynamic and biogeochemical models was
used by Žagar et al. (2007) for Hg in the Mediterranean Sea. Terrestrial Hg inputs
were based on estimated river flows and Hg concentrations derived from local
data where available and literature otherwise. Atlantic Hg inputs were based on
estimated Loop Current flows and literature estimates of Hg concentrations in
Atlantic waters. External Hg inputs and water circulation patterns were then used
as inputs to a coarse grid model of Hg cycling and bioaccumulation. A screening
level approach was employed for two reasons. First, it is often advantageous to
carry out a screening analysis to identify key processes and data requirements
required for a more extensive analysis. Second, Hg data are currently limited for
the Gulf. Literature estimates were required in some cases in the screening
analysis. It was possible to gain insights from a coarse scale modeling exercise
and available data, but a more accurate analysis is constrained until more Hg data
become available for the Gulf. Additional discussion of the hydrodynamic and
mercury cycling models used in this study is provided below.
free surface. The resulting cross-interface monthly flows were used as inputs for
the Hg model. The annual cycle of monthly flows was repeated to conduct longerterm simulations.
2.2. Hg cycling model
An existing model of Hg cycling and bioaccumulation was modified for
application to the Gulf (Fig. 1). The Dynamic Mercury Cycling Model (D-MCM) is
a time-dependent mechanistic mass balance model for Hg cycling and bioaccumulation, and is an extension of the MCM model published by Hudson et al.
(1994). D-MCM simulates inorganic Hg(II), elemental Hg(0), and MeHg in water,
sediments (solids and porewater) and a simplified food web that includes
phytoplankton, zooplankton, benthos and three fish species. MeHg dynamics in
individual fish cohorts are followed for each species using a bioenergetics
approach (Harris and Bodaly, 1998). Modifications of D-MCM from Hudson et al.
(1994) are described in the Supplementary Information (Section S6). Modifications
made to D-MCM specifically for the Gulf included accommodation of the physical
configuration of the Gulf using a multi-cell grid, and use of water circulation
patterns estimated with NCOM.
A coarse-resolution grid consisting of 19 cells was used to simulate Hg cycling
and bioaccumulation in the Gulf (Fig. 2). The grid configuration was based on the
geometry of the Gulf, water circulation, results from previous studies of cross-shelf
and along-shelf fluxes (Ohlmann et al., 2001; Morey et al., 2003), as well as
identifying regions where environmental conditions were expected to lead do
differences in Hg cycling that would translate into different Hg levels in fish.
A basic distinction was made between waters overlying the coastal shelf, and deeper
areas. Grid cells were larger in the central part of the Gulf, where it was expected
that conditions would be uniform over larger areas. Finer spatial resolution, although
Fig. 1. Conceptual diagram of Hg cycling and bioaccumulation in a model cell in
the Gulf of Mexico Hg Cycling Model.
2.1. Hydrodynamic model
Water circulation in the Gulf was simulated using the Navy Coastal Ocean
Model (NCOM), a primitive equation circulation model with a hybrid z-level
(geopotential-following) and sigma-level (terrain-following) vertical grid (Martin,
2000). Ocean velocity data were extracted from a simulation of the Gulf used by
Morey et al. (2005) to study seasonal circulation patterns along and across the
continental shelves. The simulation was configured with a horizontal resolution of
1/201 and up to 60 vertical layers (20 sigma-layers distributed over the shallow
shelf and the upper 100 m in deep water, and an additional 40 z-levels below
100 m in the deep Gulf). The model was forced by monthly climatology surface
heat and momentum fluxes derived from DaSilva et al. (1994), freshwater
discharge from 30 rivers, and flow through the Caribbean and Straits of Florida
yielding a Yucatan Current transport of approximately 27 Sv (1 Sv¼ 106 m3 s 1).
Following four years of model spin-up, 48-h horizontal currents were extracted for
six model years at each NCOM grid cell along the boundaries of the grid used for
Hg simulations with the mercury cycling model (described in Section 2.2). NCOM
uses a much higher spatial resolution than was used for the Hg screening model
analysis. Outputs from NCOM were aggregated spatially to fit the simplified layout
of the Hg screening model. NCOM velocity data were integrated along the
screening model boundaries and averaged in time to produce a monthly climatology of volume transports. Transport across cell interfaces was constrained to
ensure global volume conservation and account for small deviations in the ocean
Fig. 2. Configuration for surface cells in the Gulf of Mexico Hg screening model.
Cells 16 through 19 included 2 vertical layers separated at depth=140 m. Loop
Current location is approximate and is actually variable.
R. Harris et al. / Environmental Research 119 (2012) 53–63
still coarse, was used in coastal areas where gradients in environmental conditions
and fish MeHg exposure were expected. Cell areas ranged from 7689–425,688 km2.
The spatial domain of the hydrodynamic model simulation, and thus the Hg model,
did not explicitly include estuaries and was beyond the scope of the screening study.
Effects of estuaries were considered as described in Section 3.1 (Terrestrial Hg
Inputs).
The water column in each model cell consisted of one or two layers vertically.
Coastal cells were assumed to be well mixed in the water column out to the 140 m
isobath (the approximate depth of the shelf break) used to define the maximum
depth of these cells. The 4 model cells in the central Gulf included a surface layer
down to the 140 m depth and a deep layer that extended from 140 m to the bottom
( 4 km maximum). Dimensions of the model grid cells are given in Table S1 in the
Supplementary Information.
3. Results
3.1. External Hg loads
The Hg model required estimates of external sources of inorganic Hg(II) and MeHg, including atmospheric deposition, riverine
inputs and inflows from the Atlantic Ocean. It was assumed that Hg
inputs to the Gulf were small from hydrothermal vents (Mason
et al., this issue; Lamborg et al., 2006) and oil and gas exploration
rigs (Neff, 2002). External Hg loads from the atmosphere, rivers and
Atlantic Ocean were estimated as follows:
3.1.1. Atmospheric Hg deposition
Atmospheric Hg deposition was estimated for 2002 using simulated monthly outputs for wet and dry deposition using the
Advanced Modeling System for Transport, Emissions, Reactions and
Deposition of Atmospheric Matter (AMSTERDAM) (Vijayaraghavan
et al., 2007,2008), a version of the Community Multiscale Air Quality
(CMAQ) model (Byun and Schere, 2006). Hg deposition estimates
from AMSTERDAM were aggregated to match the coarse grid of the
Hg screening model. Simulated annual precipitation rates varied
from 0.84 to 1.92 m yr 1 among model cells. Wet deposition of Hg
was adjusted to better match average observations from the Mercury
Deposition Network (MDN) during 1997–2009 and removed an
apparent bias in over-predicted fluxes. The average wet Hg
55
deposition flux for six coastal or near-coastal MDN sites in the Gulf
region was 15.96 mg m 2 (MDN sites AL02, AL24, FL05, FL11, LZ05
and LA28). Only sites with 50 or more valid weekly measurements in
a given year were considered from 1997 through 2009 (n¼63 annual
datasets). The Gulf-wide average annual Hg wet deposition from
simulations was 22.0 mg m 2. AMSTERDAM estimates of wet deposition were therefore multiplied by 0.73 (15.96/22.0) with the resultant annual wet Hg deposition flux in model simulations averaging
15.96 mg m 2 across the Gulf (range among cells: 6.9–23.0 mg m 2).
Simulated dry Hg deposition rates were used directly with no
modification (mean annual value of 11.6, range 7.5 to 15.9 mg m 2).
Fig. 3 shows the spatial variations in simulated annual wet plus dry
deposition fluxes for total Hg (THg). Monthly estimated wet and dry
Hg deposition rates for each model cell are provided in Tables S2 and
S3 in the Supplementary Information.
Atmospheric wet deposition of MeHg was estimated by combining precipitation estimates with an assumed MeHg concentration in precipitation. Observations of MeHg in precipitation are less
common than for inorganic Hg. Hall et al. (2005) reported a range
of 0.02 to 0.23 ng MeHg L 1 for sites in the Great Lakes Region, and
Graydon et al. (2008) reported a mean value of 0.08 ng MeHg L 1
for the Experimental Lakes Area, Ontario from 1992–2006. Louis
et al. (1995) reviewed observations of MeHg concentrations in
Scandinavia and North America at that time, with concentrations
ranging fromo0.005 to 0.59 ng L 1. Guentzel et al. (1995)
reported lower concentrations (o0.005 to 0.022 ng L 1) for 11
measurements at sites in South Florida. Given the proximity of the
Guentzel et al. (1995) data, we assumed a uniform MeHg concentration of 0.04 ng L 1 for all Hg screening model cells.
3.1.2. Terrestrial Hg inputs
Terrestrial inputs of inorganic Hg(II) and MeHg were based on
riverine flows used as inputs to NCOM simulations, combined with
Hg concentrations estimated with field data or literature values.
Monthly flows for 30 U.S. rivers were derived from historic United
States Geological Survey river gauge data (USGS, 2011). Mexican
Fig. 3. Atmospheric deposition fluxes for total Hg (wet plus dry deposition) used in Gulf screening model simulations for existing conditions. Pie charts show relative
partitioning between wet (blue) and dry (red) deposition fluxes.
56
R. Harris et al. / Environmental Research 119 (2012) 53–63
river flows were derived from the Compendio Basico del Agua en
Mexico (CNA, 2001). Hg loads for the Mississippi and Atchafalaya
Rivers were based on Rice et al. (2008), who estimated 6.25 and
3.25 t yr 1 respectively, primarily associated with high concentrations of suspended solids (275–295 mg L 1). These loads correspond to overall inflow Hg concentrations in the Mississippi and
Atchafalaya Rivers of 15 ng L 1 unfiltered for the model simulations, using NCOM flows. MeHg concentrations of 0.16–0.17 ng L 1
unfiltered were estimated for these two rivers using an analogous
approach and assuming that MeHg concentrations on solids were
0.5 ng g 1 (1% of THg concentrations).
Site data for riverine Hg loads to most other regions of the Gulf
were not available. A USGS national survey of Hg in streams
(Scudder et al., 2009) estimated median concentrations of THg of
1.90 ng L 1 (THg) and 0.11 ng L 1 (MeHg) for basins without
mining activities. Allowing for the potential removal of riverine
Hg in estuaries (see below), Hg concentrations assigned for river
inputs to other model cells ranged from 1–3 ng L 1 for inorganic
Hg(II) and 0.03 to 0.10 ng L 1 for MeHg. The higher values were
assigned for the southern portion of the Florida Gulf coast (cell 1)
in consideration of Everglades Hg export. Rumbold et al. (2010)
reported concentrations of THg and MeHg ranging from 0.36 to
5.98 ng L 1 ando0.02 to 1.79 ng L 1 respectively in transects in
Florida Bay, with higher values in the mangrove transition zone.
Hg trapping in estuaries has been reported in several studies.
In the absence of Hg point sources, estuaries are typically net
sinks for inorganic Hg via a combination of settling, including
DOC coagulation and evasion (Lee et al., 2011; Choe and Gill,
2003; Benoit et al., 1998). The effects of estuaries on MeHg
concentrations are not as consistent, with examples of both net
removal and supply. Non-conservative removal of MeHg was
observed for example by Choe and Gill (2003) along a salinity
gradient in San Francisco Bay. In contrast, Rumbold et al. (2010)
found that MeHg (and THg) concentrations increased in the
mangrove transitional zone between the freshwater end member
and the open waters of Florida Bay, and estimated that sediment
Hg methylation, particularly within the mangrove transitional
zone, may exceed terrestrial runoff loadings in Florida Bay. The
effects of estuaries on inorganic Hg and MeHg supply to the Gulf
of Mexico need further study. Estimates of net riverine Hg
delivery to the Gulf of Mexico presented here are uncertain but
sufficient for the purposes of a screening analysis. A sensitivity
analysis examined the effects of varying inflowing river Hg loads
in model simulations (see Section 3.3).
3.1.3. Atlantic Hg inputs
Atlantic inputs of inorganic Hg(II) and MeHg to the Gulf were
based on estimates of inflowing water volumes and associated Hg
concentrations. The Loop Current gross annual-average inflow from
the NCOM hydrodynamic model simulation was approximately
27 Sv (1 Sv¼106 m3 s 1), which was reasonable compared to
recent estimates from observations ranging from 23.1 Sv (Candela
et al., 2003) to 30.3 Sv (Rousset and Beal, 2010). Loop Current
transport at the Yucatan Channel was governed by inflow at the
model eastern open boundary, which was relaxed to temperature
and salinity fields derived from the World Ocean Atlas 1998
(WOA98, Conkright et al., 1998) monthly climatology, and to a
normal velocity profile dynamically consistent with the WOA98
fields. Inflowing Atlantic concentrations of Hg(II) and MeHg were
estimated from the literature, as no direct measurements of
inorganic Hg(II) or MeHg in the Loop Current were available.
Sunderland and Mason (2007) reported concentrations for THg in
Atlantic waters (north, south, equatorial) that averaged 0.43 ng L 1
(n¼6). This value was used as the inflow Hg(II) concentration for
the Loop Current. An inflowing Loop Current MeHg concentration of
0.025 ng L 1 was assumed, based on observations of 0.005 to
0.04 ng L 1 in Atlantic waters (Mason and Gill, 2005) and 0.02–
0.05 ng L 1 in the North Pacific (Sunderland et al., 2009).
3.2. Model calibration
Prior to simulating Hg cycling in the Gulf, D-MCM modules
related to particle budgets and fish growth were calibrated. Particle
budgets were estimated for three types of regions: central Gulf
(cells 16–19, Fig. 2), cells receiving large inputs of solids from the
Mississippi and Atchafalaya Rivers (cells 4 and 5), and the remaining coastal cells. Long term mass sedimentation rates for cells other
than the Mississippi and Atchafalaya Rivers were calibrated in the
range of 300 g m 2 yr 1, comparable to rates reported by Yeager
et al. (2004) ranging from 200–500 g m 2 yr 1 for a set of 5 cores
collected at depths from 985 to 3560 m. Cells receiving waters from
the Mississippi and Atchafalaya Rivers were calibrated with sedimentation rates on the order of 4500 g m 2 yr- 1. Particle fluxes
used for simulations are considered first order approximations, and
the influence of particle settling rates was examined in the
sensitivity analysis (Section 3.3). Fish growth rates were calibrated
for the three fish species included in simulations. King mackerel
(Scomberomorus cavalla) was selected as the top level predatory
species, with blue runner (Caranx crysos) as an omnivore and
Atlantic thread herring (Opisthonema oglinum) as the lowest trophic
level fish species. Additional information on particle budgets and
fish growth is provided in Sections S3 and S4 of the Supplementary
Information.
To calibrate the model for Hg cycling and bioaccumulation,
observations of THg and MeHg concentrations in the Gulf were
used where possible as a guide. Hg concentration data were
available for fish in the Gulf, as were limited data for Hg in Gulf
sediments. No water column observations were available for THg
or MeHg in the Gulf, or for MeHg concentrations in the lower food
web. In these cases the model was calibrated to approximate
observations in other open ocean or coastal waters. Similarly, Hg
evasion estimates were not available for Gulf waters, and evasion
rates were calibrated to be within the range reported for other
ocean systems. Simulations were run 100 years to approach
steady state, and outputs were saved every 5 days for the 101st
year to estimate annual fluxes.
3.2.1. Inorganic Hg calibration
The model was calibrated to produce inorganic Hg(II) concentrations generally in the range of 0.3 to 0.6 ng L 1 in the surface
layer. While direct measurements of Hg concentrations in the
Gulf water column are lacking, these simulated concentrations
are comparable to observations in Atlantic waters. Sunderland
and Mason (2007) reported concentrations for THg in Atlantic
waters (north, south, equatorial) that averaged 0.43 ng L 1 (n ¼6).
Model cells receiving Hg inputs from the Mississippi and Atchafalaya Rivers were exceptions to the above range, with simulated
inorganic Hg(II) concentrations approaching 1 ng L 1, due to large
inputs of Hg from these rivers. Simulations resulted in apparent
partitioning values (kd¼particle concentration divided by filtered
concentration) of 5.1 (log 10, L Kg 1) for inorganic Hg(II) in the
water column, using default model values for Hg partitioning. No
estimates of seston Hg partitioning were found for the Gulf, but
the simulated value is comparable in order of magnitude to a
value of 5.61 (log 10) reported for Passamaquoddy Bay, an
embayment of the Bay of Fundy, Nova Scotia, by Sunderland
et al. (2010).
Simulated concentrations of inorganic Hg(II) in sediments were
in the range of 10–60 ng g 1, with higher values in coastal areas
and lower values in deepwater sediments in the central Gulf. These
R. Harris et al. / Environmental Research 119 (2012) 53–63
values are comparable to observations by Liu et al. (2009) in the
range of 5–60 ng g 1 for sediments on the coastal shelf in the
vicinity of the Mississippi and Atchafalaya Rivers. Delaune et al.
(2008) reported THg concentrations ranging from 6–58 ng g 1
(mean¼24) for sediment samples taken in Lake Borgne and
Chandeleur Sound in the Louisiana Pontchartrain Basin. Kannan
et al. (1998) reported THg concentrations ranging from 3–
219 ng g 1 (mean values) for sediments in 22 Florida estuaries
(o70 ng g 1 for 20 of 22 bays), although these locations are
outside the domain of the Hg screening model. Inorganic Hg(II)
partitioning in sediments was adjusted to produce a kd of 4.2
(log 10, L Kg 1), comparable to values of 3.1–5.0 (log 10, L Kg 1)
reported by Liu et al. (2009) in northern coastal areas of the Gulf.
3.2.1.1. Inorganic Hg fluxes. When the Gulf is viewed as a whole,
the Loop Current is estimated to account for 85–90% of the total
supply of Hg to the Gulf (Fig. 4). The Loop Current also dominates
overall losses of inorganic Hg(II) from the Gulf in model
simulations, as it exits through the Straits of Florida. Vertical Hg
fluxes across the air/water interface and sediment/water interfaces
are small compared to fluxes associated with the Loop Current.
Simulated Hg evasion rates for the Gulf averaged 11 mg m 2 yr 1,
lower in central areas ( 8 to 13 mg m 2 yr 1) and higher in
northern coastal areas ( 10 to 425 mg m 2 yr 1) where higher
inorganic Hg(II) concentrations were predicted as a substrate for Hg
reduction. These rates were achieved by adjusting the model
57
constant for Hg(II) photo-reduction to produce a Gulf-wide Hg
evasion comparable to literature estimates for oceans. Sunderland
and Mason (2007) estimated a mass transfer coefficient of
0.57 m d 1 and 13% of surface water Hg as dissolved gaseous Hg
for the Atlantic from 35 to 55 degrees latitude. This translates into
an evasion rate of 12 mg m 2 yr 1 for an Atlantic concentration of
0.43 ng L 1 (average of 6 samples reported by Sunderland and
Mason (2007)). Other reported evasion rates from oceans include:
6–9 mg m 2 yr 1 global ocean Hg evasion (Mason et al., this issue;
Soerensen et al., 2010), 24 mg m 2 yr 1 for Long Island Sound
(Balcom et al., 2004), and 20 mg m 2 yr 1 in the Mediterranean
(Rajar et al., 2007). Hg(0) concentrations in surface waters ranged
from 4–80 pg L 1 in simulations in central areas, and up to
150 pg L 1 in some coastal areas. Literature estimates of
Hg(0) concentrations in marine surface waters vary widely,
e.g.o10 to 4250 pg L 1 (Mason and Gill, 2005).
While the Loop Current dominated Hg loading to the Gulf as a
whole, water mixing was incomplete and some areas were not as
influenced by the Loop Current as others. NCOM simulations
indicated that the southern Gulf coast in Florida (cell 1) is largely
isolated from the Loop Current due to limited transport across the
wide shelf. As a result, direct inputs of atmospheric Hg were
predicted to be the dominant Hg source for this region (Fig. 5a).
River inputs of Hg to cell 1 were low due in part to low runoff
flows. Riverine Hg inputs were more important in cells along the
north coast of the Gulf, influenced by large Hg inputs from with
the Mississippi and Atchafalaya Rivers (Fig. 5b). In the central Gulf
Fig. 4. Annual model budgets for inorganic Hg(II) and MeHg for the Gulf of Mexico overall. Fluxes are expressed as mg m 2 yr 1.
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R. Harris et al. / Environmental Research 119 (2012) 53–63
Fig. 5. Simulated annual mass balances for inorganic Hg(II) and MeHg in three regions and overall in the Gulf of Mexico. Fluxes are mg m 2 cell area yr 1. Dark
shading ¼Hg sources. Light shading ¼ Hg losses.
R. Harris et al. / Environmental Research 119 (2012) 53–63
(e.g. cell 16), flows between cells represented the largest terms in
the inorganic Hg(II) budget (Fig. 5c). Hg in flows entering cell 16
could originally have been loaded to any region in the Gulf.
Simulations were carried out to establish the origin of Hg
concentrations predicted in each cell. This was accomplished
through a series of model runs where only one of the three
external Hg sources (direct atmospheric deposition, riverine
loads, or the Atlantic Ocean) was active in a given simulation.
For example, one scenario included atmospheric deposition while
riverine and Atlantic Hg loads (inorganic and MeHg) were set to
zero. It was then possible to estimate the relative contribution of
each external Hg source to predicted Hg levels in each model cell.
These simulations indicated that the relative importance of
external Hg sources varied widely among the model cells in
terms of contributing to inorganic Hg(II) concentrations in surface
waters (Fig. 6). Each of the three external Hg loads was the largest
source to at least one of the model cells. In the central Gulf the
Loop Current tended to be the largest source of inorganic Hg.
Inputs from the Mississippi and Atchafalaya River were the
largest source of Hg in coastal waters in the vicinity of these
rivers. Atmospheric deposition was predicted to be the largest
source of Hg along the Gulf coast of Florida.
3.2.2. MeHg calibration
In the absence of direct measurements of MeHg concentrations in the Gulf water column, the model was calibrated to
produce MeHg concentrations in the range of 0.02 to 0.03 ng L 1
in the surface layer, comparable to observations of 0.005 to
0.04 ng L 1 in Atlantic waters (Chen et al., 2008 review) and
0.02–0.05 ng L 1 in the North Pacific (Sunderland et al., 2009).
An important determinant of predicted MeHg concentrations was
the rate of in-situ production. The relative importance of methylation in the water column and sediments is not known however
for the Gulf, and data are lacking to infer sources using MeHg
concentration gradients. The model was therefore calibrated with
methylation occurring in both the deeper waters (below 140 m)
of the central Gulf, and in sediments in all zones. Simulated MeHg
concentrations in the deep waters of the central Gulf cells ranged
from 0.06 to 0.10 ng L 1, higher than in surface waters. This was
consistent with observations in the open Pacific by Sunderland
et al. (2009) where MeHg concentrations peaked at intermediate
depths ( 400 to 800 m) in the range of 0.08 ng L 1, while surface
concentrations were lower, 0.02 ng L 1. Cossa et al. (2009) and
59
Heimbürger et al. (2010) similarly reported higher MeHg concentrations in the Mediterranean Sea at the 100–1500 m depth range
(up to 0.08–0.10 ng L 1) compared to surface waters (0.01–
0.02 ng L 1).
Default model values for MeHg partitioning in the water column
resulted in apparent partitioning values of 4.8 (log 10, L Kg 1).
While no MeHg data were available for the Gulf water column, this
value is comparable to observations by Hammerschmidt and
Fitzgerald (2006) for microseston in Long Island Sound (log 10 kd¼
4.2, wet weight basis, higher on a dry weight basis).
Simulated concentrations of MeHg in sediments were in the
range of 0.1 to 1.0 ng g 1, with higher values in coastal areas and
lower values in deep-water sediments in the central Gulf. Surface
sediment concentrations of MeHg were reported by Liu et al. (2009)
in the range of 0.02–0.30 ng g 1 for sites on the coastal shelf in the
vicinity of the Mississippi and Atchafalaya Rivers. Delaune et al.
(2008) reported THg concentrations ranging from 0.05–0.60 ng g 1
(mean¼0.21 ng g 1) for sediment samples taken in Lake Borgne
and Chandeleur Sound in the Louisiana Pontchartrain Basin. No
data were found regarding MeHg concentrations in sediments in
deepwater areas of the Gulf. Hollweg et al. (2010) reported
sediment MeHg concentrations in sediments offshore of Chesapeake Bay in the range of 0.04 ng g 1 on the coastal shelf and
0.5 ng g 1 on the shelf slope. MeHg partitioning in sediments was
adjusted to produce apparent partitioning in the range of 3.0
(log 10, L Kg 1). While no MeHg partitioning data were available
in Gulf sediments, Hollweg et al. (2010) reported kd values (log 10,
L Kg 1) of 1.81–2.56 in coast shelf sediments and 1.79–4.16 in
slope sediments offshore of Chesapeake Bay.
Simulated concentrations of MeHg in zooplankton ranged
from approximately 50 to 100 ng g 1 dry weight. No data were
available for zooplankton MeHg concentrations in the Gulf.
Simulated MeHg concentrations in benthos ranged from 4–
40 ng g 1. No observations were available for comparison.
3.2.2.1. MeHg fluxes. Similar to the model results for inorganic
Hg(II), the Loop Current was estimated to be the largest MeHg
source to the Gulf as a whole (70–75%, Fig. 4), but mixing of
MeHg in Gulf waters was incomplete. The largest estimated source
of MeHg to the southern Gulf coast in Florida (cell 1), for example,
was sediment methylation (Fig. 5e). Riverine MeHg inputs were
Fig. 6. Predicted relative contribution of external Hg sources to inorganic Hg concentrations in surface waters. Results are averages for last year of simulation (approaching
steady state).
60
R. Harris et al. / Environmental Research 119 (2012) 53–63
more important in cells along the north coast of the Gulf, influenced
by Hg inputs associated with the Mississippi and Atchafalaya Rivers
(Fig. 5f). In the central Gulf (cell 16), in-situ water column
methylation and inflows from other cells represented the largest
source terms for MeHg (Fig. 5g). Some of the inflow MeHg load may
have originated from production within Cell 16 deep waters and
exited and returned via recirculation.
The model calibration assumed methylation in both sediments
and the water column in the deeper layer (below 140 m depth),
adjusted to rates that would produce plausible concentrations in
water and sediments. Biological demethylation was included in
sediments, but assumed to be a minor flux relative to gross
methylation. Given the uncertainty regarding the actual source of
methylation in oceans, alternative calibrations were developed to
examine the implications for MeHg concentrations if methylation
occurred only in sediments or only in deeper waters. The rate
constants for methylation were adjusted in each case to obtain
MeHg concentrations in the surface layer on the order of
0.02 ng L 1. Differences between these two scenarios emerged
for predicted deep water MeHg concentrations. The model calibration with methylation only in sediments resulted in MeHg
concentrations in deep waters that were essentially the same as
in the surface layer and Loop Current, on the order of 0.02 ng L 1.
The calibration with methylation only in deep waters resulted in
MeHg concentrations in deep waters in the range of 0.06 to
0.10 ng L 1, or 3–5 fold higher than in the surface layer.
Initial model simulations produced high rates of photochemical degradation of MeHg, when using the default rate
constant from freshwater systems. This was due in part to the
deeper penetration of light in low DOC waters. The rate constant
for MeHg photo-degradation was reduced by approximately an
order of magnitude in order to obtain marine MeHg concentrations in surface waters of 0.02 ng L 1. MeHg photo-degradation
rates averaged 0.5 mg m 2 yr 1 for the whole Gulf in the model
simulation, up to 1 mg m 2 yr 1 in some cells. This flux could
for example be achieved with an average rate constant of roughly
10% per day over a depth of 2–3 m, and MeHg dissolved
concentrations in the range of 0.005 to 0.01 ng L 1. Monperrus
et al. (2007) reported MeHg photo-degradation rates of 6–24% per
day in bottles incubated at 0.5 m depth. Whalin et al. (2007)
reported incubation rates up to 5 10–6 per second ( 40% per
day) in surface samples collected at a depth of o5 m and then
incubated onboard a ship under ambient light. The Whalin et al.
(2007) maximum rate would not apply at all times, and would
decline with depth in the water column. Whalin et al. (2007) also
suggested that the loss of MeHg in surface waters may include a
component that is not photo-chemical.
3.3. Sensitivity analysis
A ‘‘minimum–maximum’’ approach was used for the Gulf Hg
screening model to identify those inputs whose bounds of
uncertainty have the greatest degree of influence on predicted
Hg concentrations. For each input being evaluated, the model was
run twice, using high and low limits for the input, while all other
parameters were unchanged. The sensitivity index for the input
(SI) (Hoffman and Gardner, 1983) was calculated as
SI ¼ 12ðlow output result=high output resultÞ
A value of zero would mean no sensitivity of model outputs to
the input tested. The maximum possible sensitivity is 1 and is
approached asymptotically. This simple technique reflects both
classical sensitivity (the partial derivative of output Y to parameter X), and the range of variability of the input parameter. The
sensitivity index does not necessarily address model parameter
interdependencies.
Three endpoints were chosen for evaluation were (1) inorganic
Hg(II) concentration in the surface layer (unfiltered), (2) MeHg
concentration in the surface layer (unfiltered), and (3) MeHg
concentration in cohort 10 king mackerel (age 9–10 years).
Similar to base calibration simulations, sensitivity scenarios were
run for 100 years to approach steady state. During the 101st year,
outputs were saved at 5 day intervals, from which annual
averages were calculated. It was expected that results would be
systematically different for coastal cells in comparison to cells in
the central Gulf, and results were averaged for these two groups.
Twenty-eight model inputs were tested (Table S4 in the Supplementary Information). The rationale for minima and maxima for
selected inputs are proved in Table S5 in the Supplementary
Information.
Predicted concentrations of inorganic Hg(II) in surface waters
in all cells were sensitive to surface water dissolved organic
carbon (DOC) and photo-reduction rate. Some inputs affected
central cells more than coastal cells, and vice-versa. The central
cells were relatively sensitive to the concentration of inorganic
Hg(II) in the inflowing Loop Current, while coastal cells were
relatively sensitive to uncertainty regarding riverine Hg(II) loads
and inputs related to sediment characteristics.
MeHg concentrations in age 9–10 year old king mackerel were
sensitive in all cells to uncertainty associated with MeHg bioaccumulation factors (BAFs) for phytoplankton and zooplankton,
MeHg photodegradation rate, bioenergetics activity coefficient
(e.g., energy spent looking for food), and surface water DOC (Fig.
S5 in the Supplementary Information). The sensitivity of fish
MeHg concentrations to the phytoplankton BAF was strongly
associated with the wide range of values used in the minimum–
maximum simulations (3 orders of magnitude). Field data on the
appropriate BAF for MeHg in plankton will significantly reduce
the sensitivity of the model to this input, as the minimum–
maximum range will be much reduced. Fish Hg concentrations in
central cells were sensitive to uncertainty associated with MeHg
concentrations in the inflowing Loop Current, and to water
column methylation, while fish Hg levels in the coastal cells were
more sensitive to river sediment characteristics, river loads of
Hg(II) and MeHg, and benthos MeHg levels. Additional description
of the sensitivity analysis is provided in Section 5 of the Supplementary Information.
4. Discussion
The screening level model analysis resulted in estimates of Hg
concentrations and fluxes in the Gulf that are consistent with
observations from the Gulf where available, and with observations from other ocean systems for those parameters where Gulf
data were not available. The mass balance approach also ensured
that the individual components in the model analysis combine in
a consistent overall manner. While a lack of data for some key
parameters (e.g. Hg measurements in the water column) added
uncertainty to the analysis, some important findings emerged.
If the Gulf was a well-mixed system hydrodynamically, Atlantic
Hg inputs would strongly dominate Hg loading (Fig. 4), despite,
for example, estimated atmospheric Hg deposition rates that are
high in the context of wet deposition rates observed elsewhere in
the US and in Canada. This is largely due to the magnitude of the
Loop Current flow, which exceeds water inputs from the atmosphere and watersheds by more than 2 orders of magnitude. In a
well-mixed scenario, riverine and atmospheric inputs of Hg
would be of minor importance, and little could be done to reduce
Hg loads to the Gulf apart from reducing Hg concentrations in the
Atlantic. The analysis carried out here indicates, however, that a
well-mixed view of the Gulf is inappropriate. A substantial
fraction of the water entering the Gulf via the Loop Current
R. Harris et al. / Environmental Research 119 (2012) 53–63
short-circuits and exits the Gulf before fully mixing and other Hg
sources become more important, depending on location
(Figs. 5 and 6).
Spatial differences apply to Hg sinks as well as sources. Hg
evasion is much smaller than the Hg outflow from the Loop
Current on a Gulf-wide basis, but was estimated to be an
important loss mechanism in regions such as coastal areas where
the Loop Current is not as influential (Fig. 5a). Simulated evasion
rates for the Gulf are viewed as being within the range of
observations for oceans but in need of refinement when field
estimates become available for the Gulf.
Model simulations produced plausible MeHg concentrations and
fluxes with methylation occurring in both sediments and deeper
waters in the central Gulf (below 140 m). Sediment methylation
was more important in coastal areas, while water column methylation was more important in the central Gulf. The true magnitudes of
MeHg production in sediments and the water column in the Gulf are
not known. A test simulation with all MeHg production occurring in
the water column below 140 m (versus all in sediments) produced
results more consistent with observations from the Pacific and
Mediterranean, where MeHg concentrations peaked in the range
of 0.08 ng L 1 at intermediate depths, while surface concentrations
were lower, 0.01 to 0.02 ng L 1 (Sunderland et al., 2009; Cossa
et al., 2009; Heimbürger et al., 2010). Peak MeHg concentrations
could alternatively occur at intermediate depths due to releases of
MeHg associated with settling particulates that are mineralized at
these depths. Inorganic Hg(II) would be subject to the same process,
however, and observations from the Pacific (Sunderland et al., 2009)
did not show significantly higher inorganic Hg(II) concentrations at
intermediate depths. Field profiles of MeHg and inorganic Hg(II)
concentrations in the Gulf water column would help ascertain
where methylation is occurring.
MeHg concentrations simulated in king mackerel reasonably
fit observations (0.9 mg g 1 wet muscle predicted versus
1.30 mg g 1 observed for a length of 100 cm (Lowery and
Garrett, 2005)), but perhaps fortuitously. The model may have
overpredicted biomagnification occurring at the base of the food
web in plankton, but underestimated biomagnification at intermediate trophic levels leading to predatory fish. Phytoplankton
MeHg uptake was simulated assuming that all dissolved MeHg
not bound to dissolved organic carbon was available for uptake.
Gulf waters are characterized by low DOC and high chloride levels
relative to freshwaters, resulting in relatively high predicted
MeHg bioaccumulation factors (BAF¼ratio of concentration in
organism to dissolved concentration in water) for phytoplankton
and zooplankton (5.97 and 6.60 respectively, log 10 L 1 Kg).
These BAFs and resulting MeHg concentrations in zooplankton
were higher than indicated by limited data for other marine
systems. Mason et al. (this issue) reported zooplankton concentrations in marine systems ranging from 1.1–4.0 ng g 1 wet
weight. Assuming a water content of 90% for zooplankton, the
dry weight MeHg concentrations would be 11–40 ng g 1, lower
than the concentrations predicted for the Gulf, up to 100 ng g 1
dry weight.
While plankton MeHg concentrations may have been overestimated, the simulations used a relatively simplified short food
web that consisted of phytoplankton, zooplankton, benthos and
3 fish species. Additional trophic levels would be expected to
result in higher MeHg concentrations in top level predators.
Elevated king mackerel MeHg concentrations may be associated
with lower MeHg levels at the base of the food web and a longer,
more complex food web than is currently represented in the
model. This aspect of the model calibration requires field data and
further study.
The coarse model grid used was sufficient for the purposes of
this study, but an analysis at a finer spatial resolution, if coupled
61
with supporting data, would provide an improved indication of
spatial variability of different Hg sources contributing to fish Hg
levels. Atmospheric Hg inputs to cell 1 (part of Florida Gulf coast)
were estimated for example to be the largest source of Hg, while
riverine Hg inputs were low, due in part to low river flows
estimated for this cell. A finer spatial resolution in this and other
Florida coastal areas could indicate a greater importance of terrestrial inputs in areas closer to shore. In contrast, the aggregated
monthly flows across cell boundaries may have included flows in
both directions at different times within a month or at different
locations along a boundary. The potential exists for different mixing
among cells than is estimated with net monthly flows.
5. Conclusions
An existing mass balance model of Hg cycling and bioaccumulation was modified and applied to the Gulf of Mexico, coupled with
outputs from mechanistic models of hydrodynamics and atmospheric
Hg deposition. If the Gulf is viewed as a well-mixed waterbody, the
dominant source of Hg is the Atlantic Ocean. Incomplete mixing
resulted in spatial differences in the predicted relative contribution of
external Hg sources to Hg levels in water, sediments and biota. Direct
atmospheric Hg deposition, riverine inputs, and the Loop Current
were each predicted to be the most important source of Hg to at least
one of the regions in the model. While mixing associated with water
circulation was incomplete, the simulations predicted nonetheless
that sufficient mixing occurred such that fish Hg levels in any given
location in the Gulf were affected by Hg entering other parts of the
Gulf. This suggests that a Gulf-wide approach is warranted to reduce
Hg loading and fish Hg concentrations.
Although the screening model framework was sufficient for the
purposes of this study, the lack of basic observational data for a
number of critical parameters means that the model is not yet
adequately constrained to reliably predict the relationship between
Hg loading and resulting fish Hg concentrations in different Gulf
regions.
6. Future needs
Field data are critically needed to better describe THg and MeHg
levels in estuaries, coastal and pelagic regions, including measurements in the water column, sediments, and lower food web.
Without this information it will not be possible to adequately
understand the factors controlling fish Hg levels in the Gulf. MeHg
and THg concentration transects, for example, are needed vertically
and horizontally in the water column to identify gradients that
would help identify zones that are sources of MeHg.
The sensitivity analysis indicated that external Hg sources and
several processes in the Hg cycle in the Gulf are under-constrained
but influential on the relationship between Hg loading and fish Hg.
These fluxes include riverine and Loop Current Hg loads, methylation in water and sediments, Hg photo-chemistry in surface waters
(inorganic and MeHg), and Hg sedimentation. Trophic pathways for
MeHg in the Gulf also need attention to better understand which
sources of MeHg and ultimately inorganic Hg most affect fish MeHg
levels. Models with increased ability to represent trophic dynamics
and fish movement may help in this regard. Additional studies
including carbon, nitrogen and Hg isotopic analysis would help
clarify the extent to which MeHg in pelagic fish in the central Gulf is
associated with ‘‘local’’ production in deep waters versus trophic or
advective links with estuarine and coastal areas.
In terms of model structure, major estuaries should be explicitly included in the Hg modeling framework and the potential for
coastal marshes to play an important role in the delivery of MeHg
to coastal waters should be assessed. A finer model spatial and
62
R. Harris et al. / Environmental Research 119 (2012) 53–63
temporal resolution should be used when supporting data
become available, particularly in coastal and estuarine areas.
Acknowledgments
The authors would like to acknowledge the financial support
of the Florida Department of Environmental Protection, for both
the mass balance modeling and the human health risk analyses
for fish consumption in the Gulf of Mexico. Modeling support was
provided by EPRI and the Florida Electric Power Coordinating
Group. The Gulf of Mexico Alliance has also been supporting
mercury science in the Gulf of Mexico, needed to accomplish the
long term goals of this study. This publication was made possible
in part by NIH Grant number P42 ES007373 from the National
Institute of Environmental Health Sciences.
Appendix A. Supporting information
Supplementary data associated with this article can be found
in the online version at http://dx.doi.org/10.1016/j.envres.2012.
08.013.
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